The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD) and wireline logging applications, is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, may be used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Techniques for making microresistivity measurements of a subterranean formation are well known in the prior art for both wireline and LWD operations. For example, microresistivity sensors configured for use with non-conductive drilling fluid commonly include at least four electrodes (and may therefore be referred to in the art as “four terminal” devices): including a pair of spaced potential electrodes deployed between current injection electrode and return electrodes. In use, alternating current is passed between the injector and return electrodes and the potential difference (voltage drop) between the potential electrodes is measured. The formation resistivity in the region of the potential electrodes may then be calculated from the measured potential difference. Those of ordinary skill will appreciate that the formation resistivity tends to be approximately proportional to the measured difference.
While such “four terminal” measurements have been utilized commercially, there remains room for further improvement. For example, conventional four terminal sensors typically require the potential electrodes to be located between the current injection electrode and the return electrode. Relaxing this constraint may be desirable in certain applications. Moreover, there is a need to remove biases caused by a closely placed return electrode on the measurement response so that the measurement is sensitive only to the formation volume immediately around the injection and potential electrodes.